A dissolved oxygen concentration (hereinafter referred to as DO) in activated sludge in an aeration tank is generally about 2 [mg/l] during operation; however, operating at a DO of about 0 [mg/l] is beneficial in that power needed for aeration can be saved and the BOD treatment and the denitrification treatment can be performed simultaneously. In general, aeration volume control is performed by measuring DO of the aeration tank and determining an appropriate aeration volume on the basis of the measured value; however, when the DO value of the aeration tank is about 0.5 mg/l or less, it is difficult to know the accurate oxygen consumption-supply balance from the DO value and it is difficult to accurately maintain the dissolved oxygen state in the aeration tank. Thus, it has been difficult heretofore to implement low DO operation control.
As a related art concerning low DO operation control, there has been disclosed a technique of controlling DO in the aeration tank to low DO on the basis of the treated water BOD, the predicted treated water BOD value calculated from the oxygen consumption rate of sludge, and the nitrate ion concentration (for example, PTL 1).
There has been a report of a case in which airflow control is conducted by using, as a parameter, a coenzyme nicotinamide adenine dinucleotide (NADH) involved in respiration of activated sludge microorganisms so as to control DO in the aeration tank to be within the range of 0.2 mg/l to 0.6 mg/l and to thereby remove nitrogen from municipal sewage by about 75% (for example, NPL 1).
An NADH sensor is a relatively new sensor capable of measuring changes in the aerobic range to the anaerobic range not detectable with DO meters but has a drawback in that it is affected by suspended substances in the wastewater and the measured values have variation. There has been proposed a method for reducing the control error caused by variation of the NAND sensor (for example, PTL 2).
For typical activated sludge, a method of removing BOD alone or simultaneously removing BOD and nitrogen by low DO operation is a process that provides substantial benefits if it can be controlled appropriately; however, under current circumstances, when raw water has fluctuations in concentration, composition, etc., the BOD treatment and the denitrification treatment are difficult to stably maintain.
In order to accurately carry out BOD/denitrification simultaneous treatment by low DO operation, not only control through DO meters but also accurate tracking of the amount of oxygen supplied by aeration corresponding to the influent fluctuations and changes in operation conditions is critical.
The applicant of the subject application has solved these problems and conceived a method for stably controlling ultralow DO operation for BOD/denitrification simultaneous treatment, and has obtained a patent (PTL 3).